1. Field of the Invention
The invention relates to an imaging system of a microlithographic projection exposure apparatus which is specifically configured to operate with immersion.
2. State of the Art
A known means to achieve ever higher levels of resolution in microlithography, besides using projection objectives of high numerical aperture and using light of shorter and shorter wavelength, consists of introducing an immersion medium of a high refractive index, in particular a liquid, into the space between the last optical element on the image-plane side of the projection objective and the light sensitive coating layer. This technique is referred to as immersion lithography.
The known state of the art of projection objectives includes a number of diverse designs that serve to improve the handling of the immersion liquid during the scanning activity which is performed in the production of wafers and in which a relative movement occurs in particular between the last optical element and the light-sensitive coating.
The following references to examples for designs of projection objectives are presented to delineate the state of the art, albeit without any claims of completeness and without a definitive assessment of their merits.
A projection exposure apparatus is disclosed in JP 2000-058436A, which is designed so that a supplementary lens can be inserted into the interstitial space between a projection objective that is configured as a dry objective and a light-sensitive coating layer, wherein the supplementary lens can in particular have a concave curvature on the image side. An immersion fluid, for example an oil, can be introduced between the inserted lens and the light-sensitive coating.
In WO 2004/090956, a projection exposure apparatus is disclosed with a movable substrate stage which serves to hold the substrate over the projection objective and with an immersion liquid to fill the remaining space in between at least partially with immersion liquid. The intended purpose is in particular to prevent the immersion liquid from escaping (“scattering”) and to avoid impeding the movement of the substrate stage with immersion liquid conduits.
A system for immersion photolithography is disclosed in U.S. Pat. No. 6,809,794 wherein the projection objective is likewise positioned below the substrate that is to be exposed. According to the example in which the concept is implemented, there are in particular a plurality of lenses arranged in a housing with an opening at the top which is designed so that an immersion liquid which is present in a pressure region between the last lens (shown as a bi-convex lens) and the top portion of the housing will form a liquid meniscus above the projection objective or, more specifically, over the opening.